Structure and Dynamics of the Lowest Triplet State in p-Benzoquinone I. An isotope effect study on the optical absorption, emission and ODMR spectra

Abstract The results of detailed spectroscopic experiments on the lowest nπ* triplet state of p -benzoquinone- h 4 , - dh 3 , 2,6- d 2 h 2 , - d 4 and -CH 3 in mixed and isotopic mixed crystals are presented and analyzed. The origin of the lowest B 1g (nπ*) singlet-triplet transition in p -benzoquinone- h 4 (PBQ- h 4 ) is shown to be induced by asymmetric isotopic substitution and the oscillator strength of this origin is seen to be accounted for by a corresponding decrease in intensity of a level 16.9 cm −1 higher in energy in the pure PBQ- h 4 crystal. The combined oscillator strength of these close lying levels is measured and found to be almost independent of deuteration. These results are discussed in reference to the previously proposed double minimum potential model for the lowest nπ* triplet state in PBQ- h 4 and the applicability for this model is critically examined. Optical absorption experiments on heavily doped isotopic mixed crystals of PBQ- h 4 in PBQ- d 4 show hydrogen (deuterium) bounding effects between translational inequivalent molecules to be primarily responsible for the observed cluster states. These hydrogen bounding effects also induce the electronic origin of the B 1g (nπ*) triplet state in case of a translational inequivalent dimer. A detailed vibrational analysis of the phosphorescence spectrum of PBQ- h 4 in a PBQ- d 4 host crystal at 1.8 K is presented and it is shown that the unobserved origin of the B 1g (nπ*) triplet state of PBQ- h 4 is located at 18609 ± 1 cm −1 and that the inversion splitting in this lowest excited state amounts to 21 ± 1 cm −1 in this mixed crystal system. An isotope effect is study on the vibronic structure in the emission spectrum further indicates that the excited state structure of PBQ is isotope dependent. The observed large isotope effect on the ZFS parameters of the lowest triplet state of PBQ- h 4 is demonstrated to be an intramolecular phenomenon and explained as an isotope dependent spin-orbit contribution to the ZI-S parameters, induced by localization of the nπ* excitation on oxygen. Finally the dynamics of energy migration in the dilute PBQ- h 4 in PBQ- d 4 isotopic mixed crystal is probed by concentration and temperature dependent phosphorescence intensity measurements and it is suggested that trap-exciton band communication effects are of importance in this system.